Reactive cellulose and method for making same

ABSTRACT

The invention concerns reactive cellulose, i.e. cellulose having in particular a very low degree of crystallinity obtainable with a high degree of purity. Said novel reactive cellulose is particularly used as raw material for making cellulose ethers. The invention is characterized in that the reactive cellulose is substituted by organic groups according to a degree of substitution, DS, of less than 0.2, preferably between 0.04 and 0.2.

This application is a 371 of PCT/EP98/02092, filed Apr. 9, 1998.

The present invention relates to a modified cellulose with a low degreeof substitution, and to a process for manufacturing this cellulose.

The invention relates more particularly to a cellulose containingrelatively little substitution, which has improved reactivity and a lowdegree of crystallization.

Cellulose is a natural polymer present in abundance in nature, in a verywide variety of forms, such as plants (wood, cotton, flax, etc.) or evenanimals (molluscs, etc.). This natural polymer has been used for a verylong time, in particular for the manufacture of paper, textile fibres,plastics or the like.

Modified forms such as cellulose esters or cellulose ethers have alsobeen synthesized. The cellulose ester most commonly used is celluloseacetate, which is used as a plastic material for the manufacture ofmoulded or extruded articles, in the form of fibres or yarns for thetextiles sector or in cigarette filters, for example.

Cellulose ethers are cellulose derivatives which are generally solublein a solvent such as water or an organic solvent. Cellulose ethers areused in particular as thickeners, agents for controlling the fluidity orviscosity of a medium, and dispersing agents. They can also be used forthe formation of colloids or liquid crystals, or as a matrix for themanufacture of films.

Relatively little commercial development of cellulose derivatives withvery advantageous properties is performed, with the exception of only afew derivatives such as cellulose acetate or carboxymethyl cellulose.

The reason for this is that cellulose derivatives are obtained byreaction of a compound of substitution on the hydroxyl functions of thecellulose structure. In order to obtain a product which is homogeneousand in particular soluble in solvents, it is necessary to substitute allor most of the hydroxyl groups of the cellulose, and if only some ofthese groups need to be substituted, the distribution of thesubstituents should be uniform throughout the cellulose.

However, cellulose has a structure comprising crystalline portions andamorphous portions. Consequently, without treating it beforehand, it isdifficult to substitute the hydroxyl groups present in the crystallineportions.

Processes for activating cellulose have been proposed to do this. Theseprocesses have the aim of breaking the crystalline regions of celluloseto make them amorphous and accessible to substitution compounds and tomake it possible to carry out a substitution of the hydroxyl groupswhich is total or partial but distributed homogeneously.

Activating agents are generally used simultaneously with thesubstitution compounds and are solutions of hydroxides such as metalhydroxides, for instance sodium hydroxide, aqueous ammonia, amines,dimethylformamide, dimethyl sulphoxide, acetic acid and quaternaryammonium hydroxides. The activating agent most frequently used is sodiumhydroxide, which can also act as a catalyst in etherification reactions.

In order to obtain high activation and thus a dislocation of thecrystalline portions which is as complete as possible, large amounts ofactivating agent are used. Consequently, these processes require stepsfor purifying the cellulose derivatives which are an economic burden onthe preparation processes and which may partially explain the limitedindustrial and economic development of cellulose derivatives with theexception of a few.

Thus, the discovery of novel processes for manufacturing thesederivatives more economically and with a higher degree of purity mightallow a cost-effective development of these products.

One of the aims of the present invention is to overcome this problem byproposing a reactive cellulose, i.e. a cellulose having, in particular,a very low degree of crystallinity which can be obtained with a highdegree of purity. This novel reactive cellulose is useful in particularas a starting material in the manufacture of cellulose ethers.

To this end, the invention proposes a reactive cellulose substitutedwith organic groups to a degree of substitution DS of less than 0.2,advantageously between 0.04 and 0.2.

The degree of substitution DS in the cellulose industry is defined asthe average number of substituted hydroxyl groups per unit of glucoseanhydride. As each glucose anhydride unit comprises three accessiblehydroxyl groups, the maximum degree of substitution DS is equal to 3.

According to one preferred characteristic of the invention, thecellulose of the invention has a degree of crystallinity of less than10%.

Thus, the cellulose of the invention no longer comprising crystallineportions, or only a very small proportion thereof, will make it possibleto obtain cellulose derivatives without the need for a concomitant,contaminating activation step. Moreover, the cellulose derivativesobtained using the reactive cellulose of the invention have a betterdistribution of the substituents in the cellulose, this more homogeneousdistribution being reflected by an improved solubility of the cellulosederivatives for a lower degree of substitution.

According to another preferred characteristic of the invention, theorganic groups substituted with hydroxyl groups are, in particular,hydrocarbon-based groups which can comprise hetero atoms. Thus, organicgroups which may be mentioned are:

linear or branched alkyl radicals comprising from 1 to 6 carbon atoms,

aryl, alkylaryl and arylalkyl radicals,

alkyl radicals comprising polarizing functions such as a carboxyl,nitrile or hydroxyl function.

Examples of organic groups which may be suitable for the invention are:

methyl, ethyl, propyl, benzyl,

hydroxyalkyl, such as hydroxypropyl, hydroxyethyl,

carboxymethyl, cyanoethyl, sulphoethyl.

Needless to say, the celluloses of the invention can comprise organicsubstitution groups of varied nature.

A subject of the invention is also processes for manufacturing thereactive celluloses described above.

These processes consist in treating a cellulose obtained using naturalcellulose from plant sources such as wood, cotton, flax, China grass,jute, certain algae, waste from the agrifood industry, or from animal,bacterial, fungal or amoebal sources.

These natural sources of cellulose are treated with concentrated basicsolutions to remove the hemicellulose and to recover a cellulose ofsuitable purity.

According to the invention, the cellulose thus isolated is subjected toan activation step by treatment with an activating agent to make thehydroxyl groups to be substituted accessible, followed by reacting thisactivated cellulose with at least one substituting agent, such as anetherification or esterification agent, and finally recovering themodified reactive cellulose.

In a first embodiment of the process of the invention, the activatedcellulose is subjected to a step of partial removal of the activatingagent before mixing it with the substituting agent. Advantageously, theresidual weight content of activating agent after this removal step isless than 10%.

This removal is carried out either by washing or by evaporating theactivating agent, or by entrainment, for example by washing with asolvent for the activating agent, in which the cellulose is insoluble.

This step for removing the activating agent makes it possible,particularly when the activating agent is an alkaline solution, toremove a large proportion of this agent and give an amorphous cellulosewhich is not contaminated with the said agent.

The activation can be carried out with pure liquid ammonia placed incontact with the cellulose to be activated, under high pressure andtemperature, followed by activation of the cellulose either by abruptpressure reduction in the closed chamber containing theammonia/cellulose mixture, or by extraction or suction of theammonia-impregnated cellulose and abrupt depressurization of the saidcellulose. These two activation processes are described, respectively,in patent applications DE 19511061 and WO 96/30411.

The removal of the ammonia in the activated cellulose is advantageouslycarried out by evaporation to give a residual NH₃ content of less than2%.

This activation can also be carried out by treating the cellulose withan alkaline solution such as sodium hydroxide according to a processknown as “mercerization”.

After activation, the cellulose is washed with a solvent for the sodiumhydroxide such as methanol or ethanol, to give a weight concentration ofsodium hydroxide of less than 10%, advantageously between 2% and 10%.

These activated celluloses with a depleted content of activating agentare subjected to a substitution reaction by adding a substituting agent,according to operating conditions which vary depending on the nature ofthe substituting agent.

Generally, the amount of substituting agent added corresponds to thestoichiometric amount required to obtain the desired degree ofsubstitution (DS), and advantageously corresponds to a molar excess ofless than 150% relative to the said stoichiometric amount.

This reaction can be carried out in the presence or absence of acatalyst. Thus, conventional esterification catalysts will be used inthe esterification reactions.

The substituting agents are compounds comprising the organicsubstitution group defined above, and a function which reacts withhydroxyl groups. As reactive functions which are useful, mention may bemade, for example, of carboxylic, acid anhydride, acid halide, epoxy,isocyanate and halogen functions, and activated ethylenic bonds such asacrylonitrile or vinylsulphonate functions. Carbon sulphide CS₂ can alsobe used as a substituting agent, and leads to a cellulose xanthate. Assubstituting agents which are suitable for the invention, mention may bemade of:

acetic anhydride (cellulose acetate),

formic acid (cellulose formate),

sodium chloroacetate (carboxymethyl cellulose),

ethylene oxide (hydroxyethylcellulose),

propylene oxide (hydroxypropylcellulose),

alkyl halide (alkyl cellulose),

benzyl halide (benzyl cellulose),

acrylonitrile (cyanoethylcellulose),

urea (cellulose carbamate),

sodium chloroethanesulphonate (sulphoethylcellulose).

In the variant of activating cellulose with ammonia, a treatment of theactivated cellulose with a stoichiometric amount of sodium hydroxidecorresponding to the desired degree of substitution DS is carried outprior to the reaction with the substituting agent, when the lattercomprises a halide radical in the function which is to react with thehydroxyl groups.

Advantageously, the substituted cellulose obtained after reaction with asubstituting agent can be subjected to a purification step such as, forexample, a step of washing with water. This step is not obligatory andis carried out only if it is necessary in order to obtain the desireddegree of purity.

Thus, this purification step will often be unnecessary when thecellulose activated with ammonia is reacted directly with a substitutingagent.

In a second embodiment of the process for manufacturing the reactivecelluloses of the invention, the substituting agent is added to theammonia before the activation step.

This embodiment is suitable for substituting agents which are soluble inpure liquid ammonia or dispersible in pure liquid ammonia.

In addition, this substituting agent must be chemically inert withrespect to ammonia.

As a substituting agent which is suitable for this second embodiment ofthe process of the invention, mention may be made, in addition to thesubstituting agents already listed, of oxazoline, for example.

The use of these celluloses with a low degree of crystallinity is notlimited to the use described above, but can also comprise a use as apolymer matrix for the manufacture of compositions intended to be shapedby conventional moulding techniques such as injection or extrusion.

Other aims, advantages and details of the invention will emerge moreclearly in the light of the examples below, given purely forillustrative purposes and without any limiting nature, and the attachedfigures in which:

FIGS. 1a, 1 b and 1 c represent the X-ray diffraction diagrams of,respectively, an untreated cellulose, a cellulose activated by explosionwith ammonia and a benzyl cellulose in accordance with the invention,and

FIG. 2 represents an X-ray diffraction image obtained with the benzylcellulose of Example 5.

The examples below describe the preparation of various reactivecelluloses with a low degree of crystallinity according to the differentmanufacturing process variants of the invention, and different organicsubstitution groups.

The reactivity of the celluloses in accordance with the invention isdemonstrated by means of tests of reacting this cellulose with givensubstituting agents, for instance a silylating agent such as HMDZ.

These tests are performed according to the following procedures:

Silylation test: The reactivity of the cellulose derivatives ismonitored by mixing 0.5 g of the derivative to be treated with 10 ml ofhexamethylene disilazane and 1 ml of N-methylpyrrolidone (NMP). Anactive amount (100 mg) of ammonium chloride is added. The mixture isheated to 80° C. with stirring. The reaction mass swells slowly to givea viscous mass which prevents any stirring, this state indicating theend of the silylation reaction. However, the reaction is interruptedafter a period of 4.5 hours if stirring is still possible. Thereactivity is determined by observing the appearance of the solutionbefore stopping the reaction and changes to become more reactive (+/−:less reactive; +++: more reactive):

+/−: from 10 to 50% of the fibres are in the swollen state

+: less than 10% of the fibres are in the swollen state

++: solution showing turbidity

+++: clear solution

( ): reaction time if stopped before 4.5 hours

The reactivity of the celluloses in accordance with the invention isgenerally compared with that of an unmodified cellulose used in theconventional silylation processes.

EXAMPLE 1

Preparation of a Cellulose Activated with Ammonia

800 g of commercial chemical cellulose in leaf form containing about 96%of alpha-cellulose and about 8% by weight of water are cut into pieces1.3×1.3 cm in size.

These pieces are placed in a jacketed autoclave. Liquid ammonia underpressure is introduced into the autoclave via a valve.

The system is heated to 70° C. The pressure in the autoclave is about 20bar.

The system is maintained under these conditions for 60 seconds.

The cellulose is then transferred into an explosion reactor via a valve.As soon as the valve is opened, the ammonia pressure in the cellulosefalls rapidly, bringing about a kind of explosion and defibrillation ofthe cellulose.

The ammonia concentration in the cellulose is lowered to a value of lessthan 0.2% by weight relative to the cellulose by applying a reducedpressure.

EXAMPLES 2 TO 6

Preparation of a Benzyl Cellulose

The benzyl cellulose is obtained by reacting benzyl chloride withcellulose.

According to one embodiment of the process of the invention, thecellulose activated with ammonia, obtained in Example 1 and having aresidual weight concentration of ammonia represented by [NH₃], isdispersed in a sodium hydroxide solution with a weight concentrationrepresented by [Na]_(aq) (400 ml of sodium hydroxide solution per 10 gof cellulose).

The reaction medium is maintained at 20-25° C. for 1 h.

The modified cellulose is washed with ethanol after centrifugalfiltration to remove the sodium hydroxide. The weight concentration ofresidual sodium hydroxide (NaOH) in 100 g of cellulose, expressed as apercentage, is represented by [OH⁻] in Table I below.

The activated and centrifugally filtered cellulose is added as asuspension in an organic solvent (N-methylpyrrolidone) containingbenzyltrimethylammonium chloride, the weight concentration of which isrepresented by [R₄N]. The solution is placed under a reduced pressure of20 mbar at 40° C. to bring about removal of the alcohol.

After re-establishing atmospheric pressure, a solution of benzylchloride in N-methylpyrrolidone is added to the reaction mediummaintained under vigorous stirring.

After reaction for 1 hour at 40° C., the cellulose obtained is filteredoff and washed with water until a pH in the region of 7 is obtained forthe washing waters.

The reactive cellulose is then dried at 80° C. under a reduced pressureof 1 mm Hg.

Silylation tests are carried out in order to check the reactivity of themodified cellulose.

The proportions and concentrations of each reagent and the results ofthe tests are collated in the table below.

The degree of substitution (DS) is determined by infrared analysis.

TABLE I Activation of the cellulose Degree of Benzylation Activation[NH₃] [Na]_(aq) swelling [OH⁻] [R₄N] Molar ratio Results Ex. with NH₃ %% % % % RX/cell. DS Silylation 2 yes 0.46 14 347 28.9 4.2 0.9:1   0.12+++ 3 yes 0.015 6 336 15.5 3.9 1:1 <0.1 +++ 4 yes 3.05 2 310 — 3.9 2:1<0.1 ++ 5 yes 1.2 4 425 55 3.9 2:1 0.08 +++ 6 yes 1.55 1 240 1.7 4.0 2:1<0.1 +

The attached FIG. 1 represents the X-ray diffraction graphs of theuntreated cellulose (FIG. 1a), of the cellulose activated by explosionwith ammonia as described in Example 1 (FIG. 1b) and of the benzylcellulose of Example 5. These figures clearly show the totally amorphousnature of the benzyl cellulose whose degree of substitution is very low.FIG. 2 illustrates the X-ray diffraction spectrum obtained with thecompound of Example 5.

EXAMPLES 7 TO 20C

Preparation of a Cyanoethylcellulose

Cyanoethylcellulose is obtained by reacting acrylonitrile withcellulose.

This compound is obtained according to the following procedure:

Activated cellulose of Example 1 is dispersed in a basic sodiumhydroxide or aqueous ammonia solution. This mixture is maintained at 5°C. for one hour and is then stirred for 15 to 30 minutes.

Given amounts of acrylonitrile are then added to the mixture in order toobtain a given acrylonitrile/glucose anhydride unit molar ratio(Ac/R_(cell))

After stirring, the reaction medium is maintained at 45° C. for 1.5 to 3hours and then stored at low temperature (about 0° C.) for about 2hours.

The medium is neutralized with acetic acid.

The modified cellulose is recovered by filtration and washed with waterand alcohol.

The cellulose is then dried under vacuum at 80° C. Its reactivity ischecked by the silylation test described above.

The various molar ratios and the results of the reactivity tests arecollated in Table II below.

TABLE II Molar Activation [NH₃] Molar ratio ratio Ex. with NH₃ % weightSolvent Rcell./solvent ACN/Rcell DS Silylation  7 yes 0.13 NaOH-2% 1:271:1 0.063 ++(2.5)  8 yes ˜ NaOH-2% 1:25 2:1 0.105 +++  9 yes 0.05NaOH-1% 1:25 0.2:1   0.024 ++(1.5) 10 yes ˜ NaOH-1% 1:39 4:1 0.18 +++ 11yes 0.1 NaOH-0.5% 1:25 2:1 0.139 ++ 12 yes 0.1 NaOH- 1:23 2:1 0.03 +++0.25% 13 yes 1 NaOH-0.5% 1:20 2:1 0.066 + 14 yes ˜ NaOH-5% 1:17 4:10.0046 ++ 15 yes 8.1 H₂O 1:20 2:1 <0.0035 ++ 16 yes (1) NaOH-0.5% 1:102:1 0.284 +++ 17 yes (1) NaOH-0.5% 1:10 1:1 0.07 +++ 18 yes (1)NaOH-0.5% 1:10 0.5:1   0.02 +++ 19* yes ˜ — — ˜1:1  ++ 20C yes — NaOH-2%1:40 2:1 0.086 + (1) the cellulose was used directly after activationwith ammonia without pretreatment with an alkaline solution *theacrylonitrile was mixed with the cellulose before explosion bydissolving the acrylonitrile in liquid ammonia ˜negligible ammoniaconcentration

EXAMPLES 21 AND 22

Preparation of Cellulose Benzoate

In a similar manner, cellulose activated with ammonia, preparedaccording to Example 1, is mixed with a sodium hydroxide solution for 1hour at 20-25° C.

After centrifugal filtration to remove the excess sodium hydroxide, theactivated cellulose is mixed with a solvent (NMP) to allow exchangebetween the solvent and the water. The mixture is stored for 12 hours.The cellulose is again pressed to remove the contaminated NMP solvent.The cellulose is dissolved in a pure NMP solvent.

After cooling to 15° C., a benzyltrimethylammonium chloride salt isadded.

Benzoyl chloride dissolved in N-methylpyrrolidone is added to thereaction medium.

The reaction medium is maintained at 50° C. for 3 hours. It is thencooled to room temperature and stored for 12 hours.

The polymer obtained is filtered and washed with water and then withethanol, after which it is dried under vacuum at 80° C.

The reactivity of the substituted cellulose is checked by the silylationtest described above.

The concentrations of the various reagents and the results of thesilylation test are given in Table III below:

TABLE III negligible ammonia concentration Molar Activation [NH₃][Na]_(aq) ratio Ex. with NH₃ % % [R₄N] [OH⁻] RX/cell. Silylation 21 yes7.7 — 3.3 — 1:1 ++(1.5) 22 yes ˜ 1 3.0 2.4 1:1 +++(1)

EXAMPLE 23

Preparation of Cellulose Formate

An amount of 10 g of activated cellulose according to Example 1, whichhas a residual NH₃ content of less than 0.2% by weight relative to thecellulose, is washed with methanol (twice 200 ml) andcentrifuge-filtered. The methanol content is not critical for thesubsequent step, treatment with formic acid.

For this treatment, 200 ml of formic acid (technical grade, 98%) areadded to a 500 ml flask with the above activated cellulose. The mixtureis left at room temperature with stirring for 4 hours. The formic acidis then separated from the cellulose formate by filtration. Afterwashing and centrifugal filtration with methanol, and water, the productis dried under vacuum. The DS (formate) determination is carried out byalkaline hydrolysis (NaOH) and titration of the excess sodium hydroxide.The degree of substitution DS was equal to 0.2.

The cellulose formate obtained according to the process described isstable in boiling water. It degrades at and above a pH˜9. Solutions ofthe cellulose formate in dimethylacetamide solvent at concentrations of10% can be prepared.

What is claimed is:
 1. Cellulose having a degree of substitution (DS) ofthe hydroxyl groups with organic radicals of less than 0.2 and a degreeof crystallinity of less than 10%.
 2. Cellulose according to claim 1,wherein the organic radicals are residues of organic compoundscomprising hydrocarbon-based radicals which can contain hetero atoms. 3.Cellulose according to claim 2, wherein the organic radical compriseslinear or branched alkyl radicals comprising from 1 to 6 carbon atoms,aryl, alkylaryl and arylalkyl radicals, or alkyl radicals comprisingpolarizing functions.
 4. Cellulose according to claim 3, wherein theorganic radical comprises methyl, ethyl, propyl, benzyl, carboxymethyl,cyanoethyl or sulphoethyl groups or hydroxyalkyl groups.
 5. Process formanufacturing a cellulose according to claim 1, comprising: activating acellulose pulp by treatment with an activating agent, partially removingsaid activating agent in order to obtain a residual content ofactivating agent of less than 10% by weight in an activated cellulose,reacting said activated cellulose with an organic substitution compound,and optionally, removing the rest of the activating agent and the sideproducts of the substitution reaction.
 6. Process according to claim 5,wherein the organic substitution compound is added in a stoichiometricratio relative to the hydroxyl groups of the cellulose which are to besubstituted.
 7. Process according to claim 6, wherein the organicsubstitution compound is added in a molar excess of less than 150%relative to said stoichiometric amount.
 8. Process according claim 5,wherein the organic substitution compound comprises a hydrocarbon-basedradical which can comprise hetero atoms or a function which reacts withthe hydroxyl groups of cellulose, or carbon sulphide.
 9. Processaccording to claim 8, wherein the function which reacts with thehydroxyl groups comprises carboxyl, acid anhydride, acid halide, epoxy,isocyanate or halogen functions or an activated ethylenic bond. 10.Process according to claim 8, wherein the organic substitution compoundcomprises acetic anhydride, sodium chloroacetate, ethylene oxide,propylene oxide, alkyl or benzyl halides, acrylonitrile, urea or sodiumchloroethanesulphonate.
 11. Process according to claim 5, wherein theactivating agent comprises alkaline hydroxides or ammonia.
 12. Processaccording to claim 11, wherein the alkaline hydroxide activating agentsare sodium hydroxide or aqueous ammonia.
 13. Process according to claim11, wherein the activating agent is pure liquid ammonia, the cellulosebeing treated under pressure with ammonia in a confined chamber and thensubjected to an abrupt decrease in ammonia pressure in said chamber. 14.Process according to claim 13, wherein the cellulose activated withammonia is reacted with a substitution compound comprising isocyanatecompounds, urea, compounds comprising a nitrile function or compoundscomprising an activated ethylenic bond.
 15. Process according to claim14, wherein the substitution compound is placed in contact with thecellulose prior to the activation step.
 16. Process according to claim14, wherein the activating compound is dissolved or dispersed in liquidammonia.
 17. Process according to claim 13, wherein the celluloseactivated with ammonia is treated with an amount of hydroxide equal tothe stoichiometric amount of the hydroxyl groups of the cellulose to besubstituted in order to obtain the desired degree of substitution (DS);the cellulose treated with hydroxide is then reacted with a substitutioncompound comprising alkyl halides, carbon sulphide or organic compoundscomprising polarized double bonds, the product obtained optionally beingwashed to remove the hydroxide residues.
 18. Process according to claim17, wherein the hydroxide is sodium hydroxide.
 19. Process according toclaim 11, wherein the activating agent is pure liquid ammonia, thecellulose being treated under pressure with ammonia in a confinedchamber and then extracted from said chamber and subjected to an abruptdecrease in pressure.
 20. Process according to claim 5, wherein thecellulose is subjected to a step of mercerization with a hydroxide andthen to washing with a solvent for sodium hydroxide in order to obtain ahydroxide concentration of less than 10% by weight, said treatedcellulose is then reacted with a substitution compound comprising alkylhalides, carbon sulphide or organic compounds comprising at least onepolarized unsaturated bond, the substituted cellulose being subjected towashing in order to remove the residual hydroxide and the excessreaction products.